1. Field of the Invention
This invention relates to a method and apparatus for cooling steel strip, and more particularly to a method and apparatus for cooling steel strip of various thicknesses optimum for use in a cooling zone of a continuous annealing line.
2. Description of the Prior Art
Gas-jet cooling, cooled-rolls contact cooling, combinations of the above two (such as a method disclosed in Japanese Provisional Patent Publication No. 41321-1981) and several other methods have been proposed as means of cooling steel strip in continuous annealing equipment. Gas-jet cooling comprises shooting forth a jet stream of furnace atmosphere gas cooled in the cooling zone of a continuous annealing furnace against the surface of steel strip. Cooling by contact with cooled rolls is effected by bringing steel strip into contact with rolls cooled with a coolant passed along the inside perimeter of the roll body.
Using a furnace atmosphere gas and keeping the strip out of physical contact, gas jet cooling has a merit of high operational efficiency. On the other hand, cooling by contact with cooled rolls has recently attracted increasing attention because of a high cooling rate it achieves by bringing strip into direct contact with rolls.
But these methods are not without problems. With gas-jet cooling, high cooling rate becomes difficult to attain as strip thickness increases. When a thinner and wider strip is cooled with cooled rolls, a slight temperature variation across the strip width can lead to undesirable fracture or drawing on account of the low rigidity instrinsic to thinner strip. Setting aside such fracture and drawing, uniform distribution of tension across the strip width may be impaired, whereupon it becomes difficult to keep strip in uniform contact with the surface of cooled rolls widthwise, with ensuring uneven cooling.